The present invention relates to electrical stimulation of the expiratory muscles to produce cough in human patients and other mammals with spinal cord injuries resulting in paralysis of their expiratory muscles. The invention will allow human patients with such spinal cord injuries to cough periodically or as needed to prevent the occurrence of respiratory infections which have heretofore been a frequent cause of illness and even death among this patient population.
Human patients with spinal cord injury often have paralysis of a major portion or virtually all of their expiratory muscles and, therefore, lack a normal cough mechanism. As a consequence, many of these patients suffer from a markedly reduced ability to clear airway secretions. This factor contributes to the development of recurrent respiratory tract infections, a major cause of morbidity and mortality in this patient population.
Although mechanical methods exist which can increase peak expiratory air flow to improve cough effort, the degree of improvement with these methods is small. For example, the abdominal push assist maneuver involves assisting a patient's expiratory effort by applying pressure with both hands to the upper abdomen in a posterior and cephalad direction. If abdominal pressure is applied following spontaneous inspiration and glottic closure, an adequate cough pattern may be achievable. This procedure has been found to result in modest increments in peak expiratory flow (in the range of approximately 14%) over that achieved without assist and no change in total volume during the cough.
Another prior mechanical technique is mechanical insufflation-exsufflation. This is carried out by the application of positive pressure to the patient's airway followed by rapid decompression, which results in the generation of high expiratory airflows. This technique may also be effective in removing foreign bodies from the patient's airway.
Although these prior techniques may be generally effective, a need has been found for more effective methods. A major disadvantage of these prior techniques is that they are dependent upon trained personnel and provider-patient coordination. Consequently, these methods are costly and labor intensive.
Heretofore, no effective method and apparatus have been found for selectively electrically activating expiratory muscles of a human patient or other mammal by stimulation of the spinal cord roots to produce a functionally effective cough. The only known prior method for electrical stimulation of the abdominal muscles is via surface electrodes over the anterior abdominal wall. This prior technique does not result in complete activation of the abdominal muscles which creates only submaximal cough efforts and requires high stimulus intensities.
Accordingly, it is deemed desirable to develop methods and apparatus which overcome the foregoing deficiencies and others while providing better and more advantageous overall results.